Vitamin D has multiple systemic effects: bone and calcium metabolism, muscle function, insulin responsiveness, body-composition regulation, cell differentiation, and the immune system. Proper status of vitamin D is found to be related to risk reduction in hypertension, cardiac and vascular diseases, autoimmune diseases, and others. Furthermore, vitamin D supplementation resulted in improved endothelial function. Limited sun exposure may lead to vitamin D deficiency, and it may be assumed that modern life styles lead to a lack of sun exposure. Long work-days may be the primary risk factor for vitamin D deficiency. The purpose of this research is to study the effect of vitamin D treatment on multi systemic functions in young healthy men with vitamin D deficiency due to working conditions.